Method and apparatus for calibrating a magnetic resonance device and estimating a specific absorption rate

Abstract

Method for calibrating a magnetic resonance device (1) with a transmitter for generating an excitation field, comprising a first transmitting coil element (3) and at least one second transmitting coil element (3), wherein the first transmitting coil element (3) is coupled to the at least one second transmitting coil element (3), characterized in that: - for a first acquisition phase, the first transmitting coil element (3) is detuned and the at least one second transmitting coil element (3) is tuned; - during the first acquisition phase, an MR data set is acquired using the transmitter; - for a second acquisition phase, the first transmitting coil element (3) and the at least one second transmitting coil element (3) are tuned; - during the second acquisition phase, at least one further MR data set is acquired using the transmitter;and- by ​​means of a computing unit (7) based on the MR data set and at least one further MR data set, a calibration factor is determined for calculating a total voltage value at a feed point (6) of the first transmitting coil element (3) from voltage values ​​that can be measured at a measuring point (5) of an electrical supply line (8) of the first transmitting coil element (3).

Description

[0001] The present invention relates to a method for calibrating a magnetic resonance device with a transmitter for generating an excitation field, comprising a first transmitting coil element and at least one second transmitting coil element, wherein the first transmitting coil element is coupled to the at least one second transmitting coil element. The invention further relates to a method for estimating a specific absorption rate for an object under investigation during an examination with a magnetic resonance device, a magnetic resonance device, and a computer program.

[0002] During magnetic resonance imaging (MRI), high-frequency electromagnetic radiation is applied to the body of the person being examined. The tissue can heat up accordingly, depending on the specific absorption rate (SAR), i.e., the electromagnetic power absorbed per unit mass. Therefore, monitoring the SAR can be desirable during MRI scans. In parallel-transmitting MRI systems, several transmitting coils are used to generate the high-frequency excitation field. Since the fields of the individual transmitting coils can interfere differently depending on the drive signal, monitoring the SAR is particularly important in this context.

[0003] Known approaches for estimating the specific absorption rate use simulation models that simulate the entire system, including the person being examined and the various components of the magnetic resonance device, to estimate a local specific absorption rate in the relevant area from corresponding input variables. The input variables used are the power or voltage applied to the individual transmitting coil elements to actively drive the excitation field.

[0004] This necessitates the use of highly complex simulation models, which must specifically depict how the applied power or voltage is used to generate concrete electromagnetic fields via the transmitting coil elements, how the electromagnetic fields of individual transmitting coil elements interact with each other, and so on. This is disadvantageous, for example, because the relationship between the electromagnetic field generated by a transmitting coil element and the applied voltage or power depends on the specific load situation of the transmitting coil element—that is, on how the specific transmitting coil element couples precisely with the other transmitting coil elements, and what coupling results from the person being examined or other components of the MRI system.The relationship ultimately depends on the impedances that are established in the transmitting coil elements and the fields that are created by the respective voltages supplied.

[0005] The resulting high complexity of the simulation models to be used, or the limited performance of such models, leads to increased computational effort for estimating the specific absorption rate, or to increased inaccuracy of the estimated specific absorption rate. In general, this entails increased effort, for example, also with regard to the creation and validation of the simulation models.

[0006] Against this background, it is an object of the present invention to provide an improved concept for providing input variables for a simulation model for estimating a specific absorption rate during investigation using a magnetic resonance device, which can increase the accuracy of the estimation.

[0007] This problem is solved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims.

[0008] The improved concept is based on the idea of ​​determining or specifying a calibration factor, according to which voltage values ​​measured at a measuring point on the supply line of a transmitting coil element can be converted into a total voltage value at a feed point of the transmitting coil element. The total voltage value is proportional to a current flowing in the transmitting coil element, which in turn is proportional to an electromagnetic field generated by the transmitting coil element.

[0009] According to the improved concept, a method for calibrating a magnetic resonance device with a transmitter, in particular a high-frequency transmitter, for generating an excitation field is described. The transmitter comprises a first transmitting coil element and a second transmitting coil element, wherein the first transmitting coil element is coupled to the at least one second transmitting coil element, in particular by inductive coupling and / or capacitive coupling. According to the method, the first transmitting coil element is detuned for a first acquisition phase, in particular by means of a tuning device of the magnetic resonance device, and the at least one second transmitting coil element is tuned for the first acquisition phase; in particular, every second transmitting coil element of the at least one second transmitting coil element is tuned. During the first acquisition phase, an MR data set is acquired using the transmitter.For a second acquisition phase, the first transmitting coil element and at least one second transmitting coil element, in particular every second transmitting coil element, are tuned, in particular by means of the tuning device. During the second acquisition phase, at least one further MR data set is acquired using the transmitting device. A calibration factor is determined by means of a processing unit based on the MR data set and the at least one further MR data set. The calibration factor can be determined from voltage values ​​measurable at a measuring point of an electrical supply line of the first transmitting coil element to calculate a total voltage value at a feed point of the first transmitting coil element. Here and in the following, magnetic resonance is abbreviated as MR.

[0010] In other words, the calibration factor links the total voltage value at the feed-in point with the voltage values ​​measurable at the measuring point.

[0011] The magnetic resonance device is specifically designed as a parallel transmission magnetic resonance device. Such devices are also referred to as parallel-transmit systems or multi-transmit systems. The transmitting coil elements can be operated independently of one another, thus enabling a particularly flexible and precise definition of the excitation field generated by the transmitting device.

[0012] Magnetic resonance imaging (MRI) and other methods for magnetic resonance-based investigation are based on the concept of aligning the nuclear spins of the object under investigation using a homogeneous background magnetic field and exciting them to precess using an excitation field, which contains a time-varying electromagnetic and therefore, in particular, a magnetic field. Resonances of the precessing spins can then be detected by means of a detection device within the magnetic resonance apparatus. The transmitting coil elements can be part of the detection device, or the detection device can contain separate coil elements.

[0013] The excitation field is usually also referred to as a high-frequency field, i.e., RF field or HF field. The excitation field is generated in particular in the form of one or more successive high-frequency pulses, i.e., RF pulses or HF pulses.

[0014] The feed point, also known as the feed port, of the first transmitting coil element corresponds specifically to a connection of the first transmitting coil element for the electrical supply line, which in turn is connected to an RF generation system. This system can generate the RF pulses or excitation pulses or feed them into the first transmitting coil element via the supply line. The connection of the first transmitting coil element to the electrical supply line necessarily results in signal reflections, so that parts of the injected excitation pulses can be reflected. Consequently, in a given situation, a forward-propagating signal component as well as a backward-propagating or reflected signal component are present in the supply line.Forward propagation refers to the direction from the RF generation system towards the first transmitting coil element, and reverse propagation refers to the opposite direction. These two signal components superimpose, so that different phases between the forward and reverse signal components can occur at the injection point and at the measurement point, which is located spatially remote from the injection point.

[0015] While the total voltage value at the injection point—that is, the sum of the forward voltage value of the forward-propagating signal component and the reverse voltage value of the reverse-propagating signal component at the injection point—is proportional to the current flowing through the first transmitting coil element and, consequently, to the electromagnetic field generated by the first transmitting coil element, this generally does not hold true at the measuring point. However, the fundamental relationship between the total voltage value at the injection point and the measured voltage values ​​at the measuring point is known.

[0016] In particular, the total voltage value is given by the following relationship: U=α*UM,v+(1 / α)*UM,r, where U denotes the total voltage value, α the calibration factor, U M,v the voltage value of the forward-propagating signal component at the measuring point and U M,rdenotes the voltage value of the backward-running signal component at the measuring point.

[0017] The calibration factor, which is given in particular by a complex number, is constant regardless of the load situation and, in particular, does not depend on the voltage values ​​measurable at the measuring point. Once the calibration factor has been determined, the total voltage value can always be calculated from the measured voltage values, even in deviating situations, using the above relationship. This total voltage value, in turn, can be used as an input for a simulation model to estimate the specific absorption rate, due to its proportionality to the current through the first transmitting coil element.

[0018] The improved concept thus removes the load dependency of the relationship between input power and generated electromagnetic field from the simulation and replaces it with the determination of the calibration factor. The dependency on properties of the transmitting coil elements, such as their ohmic resistance, as well as the properties of the coupling between the transmitting coil elements, no longer needs to be represented by the simulation model. Since the calibration factor, as described, is based on MR datasets and, if applicable, measured voltage values, this provides a more reliable input for the simulation model, enabling more accurate simulation results and potentially requiring less computational effort.

[0019] The fact that the first transmitting coil element is coupled to at least one second transmitting coil element can be understood as meaning that the first transmitting coil element is coupled to one or more second transmitting coil elements of the at least one second transmitting coil element. This coupling can be inductive, in particular. A changing electromagnetic field generated by a transmitting coil element thus induces a voltage in the respective coupled transmitting coil element, and vice versa. Alternatively or additionally, capacitive coupling is also possible.

[0020] To acquire the MR dataset or further MR datasets using the transmitter during the first and second acquisition phases, one or more excitation pulses are generated by the transmitter, specifically by one or more of the transmitting coil elements, and applied to the object under investigation. In response to the excitation pulses, MR signals are acquired by the acquisition unit of the magnetic resonance device, depending on the pulse sequence used. Based on these MR signals, the MR dataset or one of the further MR datasets is generated, particularly by the processing unit. The transmitter and acquisition units may share some components, allowing transmitting coil elements to also function as receiving coil elements.

[0021] The detuning or tuning of a transmitting coil element can be understood here and in the following as resonant detuning or tuning, meaning that the respective resonant frequency of the corresponding transmitting coil element is detuned or tuned, particularly with respect to a frequency, especially a carrier frequency, of the excitation field. Accordingly, no voltage is generated in a detuned transmitting coil element, especially the first transmitting coil element detuned during the first acquisition phase, due to coupled secondary transmitting coil elements. The detuned transmitting coil element is, in a sense, decoupled from the other transmitting coil elements. A detuned transmitting coil element is also not actively driven here and in the following; that is, no voltage or power is supplied to it.However, it cannot be concluded from this that a tuned transmitting coil element is necessarily actively driven. Rather, induced voltages can also build up in a tuned transmitting coil element that is not actively driven, due to coupling.

[0022] The fact that a transmitting coil element is detuned can be understood here and in the following as meaning that it becomes detuned if it was tuned before the detuning, or remains detuned if it was already detuned. The same applies analogously to tuning a transmitting coil element.

[0023] During the entire first acquisition phase, the first transmitting coil element remains detuned, while at least one second transmitting coil element remains tuned. Similarly, the first transmitting coil element and at least one second transmitting coil element remain tuned throughout the entire second acquisition phase.

[0024] Accordingly, at least one second transmitting coil element is actively driven during the first acquisition phase to acquire the MR data set as described. During the second acquisition phase, the first transmitting coil element and / or one or more of the second transmitting coil elements are actively driven to acquire at least one further MR data set. The second acquisition phase does not necessarily occur after the first acquisition phase.

[0025] According to at least one embodiment of the method for calibrating a magnetic resonance device according to the improved concept, every second transmitting coil element of the at least one second transmitting coil element is actively driven to acquire the MR data set, in particular during the first acquisition phase.

[0026] As explained above, the first transmitting coil element is not actively driven, as it is detuned during the first acquisition phase. Therefore, since the first transmitting coil element does not generate an electromagnetic field during this phase, either through active driving or coupling with at least one other transmitting coil element, it is ensured that the MR data set is completely unaffected by the first transmitting coil element or its properties.

[0027] The first MR data set can therefore be considered a reference data set with which at least one further MR data set can be compared in order to quantify a respective contribution attributable to the first transmitting coil element.

[0028] According to at least one embodiment, the second acquisition phase includes an acquisition step assigned to the first transmitting coil element. During the acquisition step assigned to the first transmitting coil element, a further MR data record of the at least one further MR data record is acquired, wherein the first transmitting coil element is actively driven during the acquisition step assigned to it and no second transmitting coil element of the at least one second transmitting coil element is actively driven.

[0029] Since both the first transmitting coil element and at least one second transmitting coil element are tuned during the second acquisition phase, the electromagnetic field generated during the acquisition step assigned to the first transmitting coil element is transmitted back to the first transmitting coil element and, via coupling, to at least one of the second transmitting coil elements.

[0030] According to at least one embodiment, the second acquisition phase for each second transmitting coil element of the at least one second transmitting coil element includes an acquisition step assigned to the respective second transmitting coil element. During the acquisition step assigned to the respective second transmitting coil element, a further MR data record of the at least one further MR data record is acquired, wherein the respective second transmitting coil element is actively driven during the acquisition step assigned to it, whereas no further second transmitting coil element of the at least one second transmitting coil element is actively driven, and the first transmitting coil element is also not actively driven.

[0031] In other words, during each acquisition step of the second acquisition phase, exactly one transmitting coil element of the transmitter is actively driven. Either the first transmitting coil element is actively driven, or exactly one of the second transmitting coil elements. If N-1 denotes the number of second transmitting coil elements, then N further MR data sets are generated in the second acquisition phase, with exactly one transmitting coil element being actively driven for the generation of each of the N further MR data sets—namely, the first transmitting coil element or exactly one of the second transmitting coil elements.

[0032] Since the MR dataset was acquired during the first acquisition phase without any contribution from the first transmitting coil element, each value of the MR dataset can be expressed as a linear combination of corresponding values ​​from the subsequent MR datasets. A value can, for example, correspond to the value of a corresponding pixel or voxel in an MR image, a value for a specific point in k-space, and so on. The specific content of the MR datasets is not crucial for the improved concept. In particular, it is not necessary for the MR datasets or the subsequent MR datasets to correspond to complete MR images.

[0033] The MR datasets and the other MR datasets can correspond to parts of MR images or parts of k-space datasets. The only requirement is that the values ​​of the MR datasets and the other MR datasets generally change depending on a change in the excitation field.

[0034] Utilizing the described linear relationship, the forward voltage value of the forward-propagating signal component at the measurement point can be determined based on the MR data set and at least one other MR data set. In other words, corresponding forward voltage values ​​can be determined that would be necessary to generate the MR data set from the first acquisition phase if all transmitting coil elements were resonantly tuned. This can be done, for example, by solving a corresponding system of linear equations. The reverse voltage value of the reverse-propagating signal component at the measurement point can be determined by actively driving the first transmitting coil element according to the determined forward voltage value and measuring the resulting reverse voltage value at the measurement point.Alternatively, the reverse voltage value can be determined by the processing unit based on the determined forward voltage value and a predefined scattering matrix. The scattering matrix describes the behavior of the injected excitation pulses at the transition of the connection power to the injection point.

[0035] Since the current through the first transmitting coil element, as well as the total voltage value at the injection point, must be zero due to the decoupling of the first transmitting coil element during the first acquisition phase, and thus no MR-effective field is generated by the first transmitting coil element, the calibration factor can be calculated from the relationship between the total voltage value and the voltage values ​​measured or measurable at the measuring point.

[0036] Such embodiments have the advantage that a systematic determination of the calibration factor is possible without having to explicitly compare the other MR data sets with the MR data set to identify the decoupled state. In the same way, the corresponding calibration factors can also be determined for all second transmitting coil elements of the transmitting device.

[0037] According to at least one embodiment of the method, a second transmitting coil element of the at least one second transmitting coil element is actively driven during the first acquisition phase to acquire the MR data set.

[0038] During the second acquisition phase, the first transmitting coil element is actively driven, whereby an input power, and thus an input voltage, is iteratively changed until the MR data set is reproduced by a corresponding further MR data set of at least one further MR data set.

[0039] If, with the first transmitting coil element correctly tuned, the MR data set from the first acquisition phase is reproduced in the second acquisition phase, it is ensured that no electromagnetic field is generated by the first transmitting coil element. Once this condition is established, the voltage values ​​at the measuring point can be measured, and the calibration factor can be calculated based on these values ​​as described above.

[0040] According to at least one embodiment of the method, the voltage values ​​measurable at the measuring point include the forward voltage value of the forward-propagating signal component at the measuring point and the reverse voltage value of the backward-propagating signal component at the measuring point, or the voltage values ​​measurable at the measuring point are given by the forward voltage value and the reverse voltage value.

[0041] To measure the forward and reverse voltage values, a directional coupler can be used, which is connected to or integrated into the electrical supply line at the measuring point. This allows the forward and reverse signal components to be viewed and measured separately.

[0042] According to at least one embodiment, the total voltage value is given by the forward voltage value, the reverse voltage value, and the calibration factor.

[0043] In particular, the total voltage value is equal to the sum of the forward voltage value of a forward-moving signal component at the injection point and the reverse voltage value of a backward-moving signal component at the injection point.

[0044] According to at least one embodiment, the forward voltage value is determined by the computing unit based on the MR data set and at least one further MR data set.

[0045] This can be done in particular by solving the linear system of equations as described above, or by directly comparing or aligning the MR dataset with the respective further MR dataset of the corresponding iteration, if the iterative procedure described above is used.

[0046] According to at least one embodiment, the reverse voltage value is determined by the computing unit based on the determined forward voltage value and a predetermined scattering matrix.

[0047] The scattering matrix describes the behavior of an injected voltage pulse at the transition between the supply line and the injection point. Such designs have the advantage that the reverse voltage value does not necessarily have to be measured for every required case.

[0048] According to at least one embodiment, the first transmitting coil element is actively driven according to the determined forward voltage value and the reverse voltage value is measured at the measuring point.

[0049] According to the improved concept, a method for estimating the specific absorption rate of an object under investigation using a magnetic resonance device is also described. The device includes a transmitter for generating an excitation field. The transmitter comprises a first transmitting coil element and at least one second transmitting coil element, wherein the first transmitting coil element is coupled to the at least one second transmitting coil element, in particular inductively and / or capacitively. According to the method for estimating the specific absorption rate, a method for calibrating a magnetic resonance device according to the improved concept is performed. An excitation pulse is applied to the object by means of the transmitter, and the voltage values ​​measurable at the measuring point are measured during the application of the excitation pulse.The processing unit determines the total voltage value based on the measured voltage values ​​and the calibration factor. This determined total voltage value is then used by the processing unit as input for a predefined simulation model to estimate the specific absorption rate.

[0050] The voltage values ​​are measured at the measuring point, in particular using a directional coupler.

[0051] The computing unit applies the simulation model depending on the input variable to determine the specific absorption rate for the object or a local specific absorption rate for a sub-area of ​​the object.

[0052] According to the improved concept, a magnetic resonance device is also described. The magnetic resonance device comprises a transmitter comprising a first transmitting coil element and at least one second transmitting coil element, wherein the first transmitting coil element is coupled to the at least one second transmitting coil element, in particular inductively and / or capacitively, and the transmitter is configured to apply an excitation pulse to an object under investigation. The magnetic resonance device comprises a measuring device configured to measure voltage values ​​at a measuring point of an electrical supply line for the first transmitting coil element during the application of the excitation pulse.The magnetic resonance device includes a processing unit configured to determine a total voltage value at the injection point of the first transmitting coil element, based on the measured voltage values ​​and a predefined calibration factor. The processing unit is configured to use this total voltage value as input for a predefined simulation model to determine a specific absorption rate for the object.

[0053] The magnetic resonance device, in particular a directional coupler of the magnetic resonance device, is connected to or integrated into the supply line in order to measure the voltage values ​​at the measuring point.

[0054] The calibration factor was determined in particular by means of a method for calibrating a magnetic resonance device according to the improved concept and stored on a storage element of the magnetic resonance device, in particular the computing unit.

[0055] In particular, the magnetic resonance device is configured to perform a procedure for calibrating the magnetic resonance device according to the improved concept and to store the calibration factor determined thereby on the storage element.

[0056] According to at least one embodiment of the magnetic resonance device, the measuring device includes a tuning device configured to detune the first transmitting coil element and tune the at least one second transmitting coil element for a first acquisition phase. The magnetic resonance device, in particular the transmitting device, a detection device of the magnetic resonance device, and the processing unit, are configured to acquire one MR data set during the first acquisition phase. The tuning device is configured to tune the first transmitting coil element and the at least one second transmitting coil element for a second acquisition phase. The magnetic resonance device, in particular the transmitting device, the detection device, and the processing unit, are configured to acquire at least one further MR data set during the second acquisition phase.The computing unit is designed to determine, based on the MR data set and at least one other MR data set, the calibration factor for calculating the total voltage value at the feed point of the first transmitting coil element from voltage values ​​that can be measured at the measuring point.

[0057] According to at least one embodiment, the magnetic resonance device includes a directional coupler arranged on the supply line to separate a forward-propagating signal component from a backward-propagating signal component at the measuring point. The measuring device includes a measuring instrument configured to measure a forward voltage value of the forward-propagating signal component and a backward voltage value of the backward-propagating signal component in order to measure the voltage values ​​at the measuring point. In other words, the voltage values ​​measured at the measuring point include, or are determined by, the forward voltage value and the backward voltage value.

[0058] Further embodiments of the magnetic resonance device according to the improved concept follow directly from the various embodiments of the method for calibrating a magnetic resonance device according to the improved concept and from the various embodiments of the method for estimating a specific absorption rate according to the improved concept, and vice versa. In particular, the magnetic resonance device according to the improved concept can be configured or programmed to perform a method for calibrating a magnetic resonance device according to the improved concept and / or a method for estimating a specific absorption rate according to the improved concept, or the magnetic resonance device performs such a method.

[0059] According to the improved concept, a computer program with instructions is specified. When the instructions or the computer program are executed by a magnetic resonance device according to the improved concept, in particular by the processing unit of the magnetic resonance device, the instructions cause the magnetic resonance device to perform a procedure for calibrating a magnetic resonance device according to the improved concept or a procedure for estimating a specific absorption rate according to the improved concept.

[0060] According to the improved concept, a computer-readable storage medium is also specified that stores a computer program according to the improved concept.

[0061] Computer programs and computer-readable storage media can each be understood as computer program products.

[0062] The invention will be explained in more detail below with reference to specific embodiments and associated schematic drawings. These show: Fig. 1 a schematic representation of an exemplary embodiment of a magnetic resonance device according to the improved concept; Fig. 2. A flowchart of an exemplary embodiment of a method according to the improved concept; and Fig. 3 a flowchart of another exemplary embodiment of a method according to the improved concept.

[0063] In Fig. Figure 1 schematically illustrates an exemplary embodiment of a magnetic resonance device 1 according to the improved concept. The magnetic resonance device 1 comprises a scanner tube 11, also referred to as a scanner bore, and a ground field generation device (not shown) to generate a homogeneous ground magnetic field, also referred to as B0, inside the scanner tube 11. The magnetic resonance device 1 also comprises a transmitter device to generate high-frequency excitation pulses. The transmitter device contains a plurality of transmitter coil elements 3, which are coupled to one another, for example inductively, such that each transmitter coil element 3 is coupled to at least one other transmitter coil element 3.

[0064] The magnetic resonance device 1 also includes an RF power supply system 2 for actively driving the transmitting coil elements 3 and thereby generating the corresponding excitation pulses. The RF power supply system 2 comprises a signal generator stage 10 and an amplifier stage 9. Furthermore, the RF power supply system 2 includes a corresponding electrical supply line 8, for example a coaxial cable, for each transmitting coil element 3, which connects a feed point 6 of the respective transmitting coil element 3 to the amplifier stage 9. In particular, the amplifier stage 9 can include an amplifier unit for each of the transmitting coil elements 3, which is connected to the corresponding transmitting coil element 3 via the respective supply line 8. The signal generator stage 10 can also, in various embodiments, include a corresponding generator unit for each of the transmitting coil elements 3.

[0065] To generate the excitation pulses, the transmitting coil elements 3 can therefore be individually and independently controlled via the signal generator stage 10 and the amplifier stage 9 and thus actively driven by feeding in a corresponding RF pulse via the corresponding electrical supply line 8.

[0066] The magnetic resonance device 1 also has a control system 7 which can be coupled to the amplifier stage 9 and the signal generator stage 10 in order to control them accordingly.

[0067] The magnetic resonance device 1 also includes a detection device (not shown) for detecting MR signals generated by corresponding nuclear magnetic resonances in response to the applied RF pulses. The detection device can include one or more separate detection coil elements. Alternatively or additionally, the transmitting coil elements 3 can also serve as detection coil elements. The control system 7 includes, in particular, a processing unit that is coupled to the detection device to receive the MR signals.

[0068] Furthermore, the magnetic resonance device 1 includes a measuring device 4, which is arranged at respective measuring points 5 of the supply lines 8 in order to measure the corresponding voltage values ​​of a respective forward-propagating signal component and a respective reverse-propagating signal component in each of the supply lines 8. For this purpose, the measuring device 4 can have a correspondingly arranged directional coupler for each of the supply lines 8. The measuring device 4 and the directional couplers are also connected to the control system 7, in particular to the processing unit, so that the processing unit can receive the measured voltage values ​​of the forward-propagating and reverse-propagating signal components.

[0069] The function of the magnetic resonance device 1 is explained in more detail below with reference to exemplary embodiments of methods for calibrating a magnetic resonance device according to the improved concept or methods for estimating a specific absorption rate according to the improved concept, in particular with reference to the Fig. 2 and Fig. 3.

[0070] In the following, the forward voltage value refers to the voltage value of the forward-propagating signal component, and the reverse voltage value refers to the voltage value of the reverse-propagating signal component. The described methods are also based on the following assumptions. It is assumed that at a defined reference plane of a transmitting coil element 3, the sum of the forward voltage value and the reverse voltage value is proportional to the current in the corresponding transmitting coil element 3. This applies in particular at the respective feed point 6. Furthermore, it is assumed that the current distribution within a transmitting coil element 3 is independent of the specific load distribution. The current in the transmitting coil element 3 is, in turn, proportional to the magnetic and electric field generated by this transmitting coil element 3, and the proportionality factor is also independent of the specific load situation.It is further assumed that the magnetic field generated by the transmitting coil elements 3, also referred to as B1, or a portion of the magnetic field, for example a positively circularly polarized portion, for example referred to as B1+, is proportional to the magnetic fields generated by the transmitting coil elements 3, and that the proportionality factor is also independent of the load situation. It is further assumed that a detuned transmitting coil element 3 does not generate an electromagnetic field.

[0071] Under these assumptions, the sum of the forward and reverse voltage values ​​of a given transmitting coil element 3 in a given but not necessarily known reference plane is, for example, proportional to the magnetic field component B1+ generated by this transmitting coil element 3 and proportional to the corresponding electric field. It can be assumed without loss of generality that the proportionality factor is greater than zero.

[0072] Since the reference plane on which the proportionality is given is not necessarily known, one goal of a calibration procedure according to the improved concept is to find a complex calibration factor α such that U=α*UM,v+(1 / α)*UM,r∼B1+∼E

[0073] Where U is referred to as the total voltage value and U M,v corresponds to the forward voltage value measurable at measuring point 5, as well as U M,rthe reverse voltage value measurable at measuring point 5. If the calibration factor α is known, the total voltage value for each measured combination of U can be determined. M,v and U M,r can be calculated. Since the total voltage value is then proportional to the magnetic field B1+ or the electric field, the total voltage value U can be used to simulate the specific absorption rate of an object being examined with the magnetic resonance device 1.

[0074] To determine the value of α, according to the improved concept, for example, a situation is reproduced in which, for a given transmitting coil element 3, the generated field is equal to zero, where U M,v is different from zero, so that α*UM,v+(1 / α)*UM,r=0, and α=(−UM,r / UM,v)1 / 2.

[0075] In other words, a situation is simulated in which the fields generated by the transmitting coil element under consideration disappear.

[0076] In Fig. Figure 2 shows a flowchart of an exemplary embodiment of a method for estimating a specific absorption rate according to the improved concept, wherein the method includes an exemplary embodiment of a method for calibrating the magnetic resonance device 1, which is represented by the method steps S1 to S5.

[0077] The procedure is described below for a specific transmitting coil element 3. The procedure can be carried out analogously for all other transmitting coil elements 3. The transmitting coil element 3 under consideration is also referred to as the first transmitting coil element, and the other transmitting coil elements 3 are also referred to as the second transmitting coil elements.

[0078] In step S1, the first transmitting coil element 3 of the magnetic resonance device 1 is detuned using the tuning device, and the second transmitting coil elements 3 are each tuned to resonance. In this state, an MR data set is acquired, with at least one of the second transmitting coil elements 3 being actively driven. The MR data set then contains no signals generated by the first transmitting coil element 3, since it was detuned.

[0079] In step S2, all transmitting coil elements 3, i.e., the first transmitting coil element 3 and the second transmitting coil elements 3, are tuned to resonance, and the measurement described with respect to S1 is repeated several times. Since all transmitting coil elements 3 are now tuned to resonance, the first transmitting coil element 3 generates a field in this situation even if it is not actively driven, because it is coupled to at least one of the second transmitting coil elements 3. In each iteration, a corresponding additional MR data set is generated in step S3 and compared with the MR data set from step S1. If these do not match within predefined tolerances, the forward voltage value of the first transmitting coil element 3 is adjusted, and step S2 is repeated. These iterative steps are performed until the MR data set from step S1 is reproduced. If this occurs, the first transmitting coil element 3 does not generate a field, even though the forward voltage value is not zero.The first transmitting coil element 3 was thus placed in a completely decoupled state.

[0080] In this situation, in step S4, the forward voltage value and the reverse voltage value are measured, for example, at measuring point 5 using the measuring device 4, and in step S5, the calibration factor α can be determined based on the measured voltage values ​​according to the equation above.

[0081] In the same way, the corresponding calibration factors for all other transmitting coil elements 3 can be determined.

[0082] If the object is now examined using the magnetic resonance device 1, the respective forward voltage values ​​and the respective reverse voltage values ​​at the measuring point 5 can be measured for each of the transmitting coil elements 3 in step S6 during the application of a corresponding excitation pulse and, since the calibration factors α are known, the respective total voltage value can be calculated.

[0083] In step S7, the total voltage values ​​are then passed as input to a predefined simulation model via the processing unit. Based on this input, the simulation model calculates a local specific absorption rate in a target area of ​​the object. Known simulation models can be used for this purpose. For example, commercial software products for electromagnetic simulations are known that can simulate the specific absorption rate once for each transmitting coil element. The simulation model itself is not the subject of the present invention. Rather, the present invention aims to provide the most suitable input variables possible in order to improve the estimation of the specific absorption rate in this way.

[0084] In Fig. Figure 3 shows a flowchart of another exemplary embodiment of a method for estimating a specific absorption rate according to the improved concept, wherein steps S1' to S5' again correspond to an embodiment of a method for calibrating the magnetic resonance device 1 according to the improved concept.

[0085] In step S1', a corresponding MR data set is generated for each transmitting coil element 3, whereby for each of the MR data sets generated in this way exactly one transmitting coil element 3 is detuned and all other transmitting coil elements 3 are resonantly tuned and actively driven.

[0086] In step S2', another MR data set is generated for each transmitting coil element 3, this time with all transmitting coil elements 3 being resonantly tuned, but for each of the further MR data sets, only exactly one of the transmitting coil elements 3 is actively driven. In alternative implementations, the same information can be obtained by recording linearly independent drive signals and then calculating back to the individual channel drive signal.

[0087] In order to determine the calibration factor α for one of the transmitting coil elements 3, the following system of linear equations is now solved in step S3' using the computing unit: D1=M*V.

[0088] Here, D1 corresponds to a vector containing, for example, the MR signal intensities for each considered voxel of the MR dataset from step S1' in which the considered transmitting coil element 3 was detuned during its generation. M corresponds to a matrix whose columns contain, for example, vectors with MR signal intensities for each considered voxel of the MR datasets from step S2'. V corresponds to the unknown vector being sought. The entries of vector V are given by construction by the respective forward voltage values ​​of the individual transmitting coil elements 3.

[0089] After the linear system of equations has been solved, the reverse voltage value for the considered transmitting coil element 3 can also be measured in step S4' or calculated based on a scattering matrix, so that the respective calibration factor α can be calculated in step S5' as described above. This can be done in the same way for all other transmitting coil elements 3.

[0090] Steps S6 and S7 correspond to those concerning Fig. 3 were described.

[0091] The method can be further simplified in various embodiments. For example, it is not necessary to generate a full MR image for each MR dataset. It is generally sufficient to acquire enough data to solve the linear system of equations. This can be achieved with just a few lines in k-space or, for example, using a single FID. Furthermore, the spatial coding capabilities of the acquisition device can also be used to obtain additional information about the field distribution.

[0092] Furthermore, it is not always necessary to measure the forward and reverse voltage values, as the forward voltage can sometimes be predicted from known system properties. The reverse voltage can, if necessary, be determined from the forward voltage based on a predefined scattering matrix.

[0093] The MR datasets can be of various types. For example, they can be acquired using gradient spin echo (GSE) or echo planar (EPI) techniques, or they can be B1 maps. In general, any data dependent on MR signals that exhibit a significant dependence on the applied excitation fields can be used.

[0094] As described in particular with regard to the figures, the improved concept provides a way to generate input variables for simulation models used to estimate the specific absorption rate, enabling a more accurate estimation. In various embodiments, the measurements at the directional coupler are directly adjusted for this purpose, thus eliminating the need for assumptions about the transmitting coil elements or comparative measurements. By measuring the fields virtually directly, the associated uncertainties can be avoided.

Claims

[1] Method for calibrating a magnetic resonance device (1) with a transmitter for generating an excitation field, comprising a first transmitting coil element (3) and at least one second transmitting coil element (3), wherein the first transmitting coil element (3) is coupled to the at least one second transmitting coil element (3), characterized by , that - for a first acquisition phase the first transmitting coil element (3) is detuned and at least one second transmitting coil element (3) is tuned; - during the first acquisition phase, an MR data set is acquired using the transmitting device; - for a second acquisition phase the first transmitting coil element (3) and at least one second transmitting coil element (3) are tuned; - during the second acquisition phase, at least one additional MR data set is acquired using the transmitting device; and - a calibration factor for calculating a total voltage value at a feed point (6) of the first transmitting coil element (3) is determined by means of a computing unit (7) based on the MR data set and at least one further MR data set from voltage values ​​that can be measured at a measuring point (5) of an electrical supply line (8) of the first transmitting coil element (3). [2] Method according to claim 1, characterized by , that to acquire the MR data set each of the at least one second transmitting coil element (3) is actively driven. [3] Method according to claim 2, characterized by , that - the second acquisition phase contains an acquisition step assigned to the first transmitting coil element (3); - during the acquisition step assigned to the first transmitting coil element (3), another MR data set of at least one further MR data set is acquired, wherein the first transmitting coil element (3) is actively driven and none of the at least one second transmitting coil element (3) is actively driven. [4] Method according to claim 3, characterized by , that - the second acquisition phase for each of the at least one second transmitting coil element (3) contains an acquisition step assigned to the respective second transmitting coil element (3); and - during the acquisition step assigned to the respective second transmitting coil element (3), a further MR data set of at least one further MR data set is acquired, wherein the respective second transmitting coil element (3) is actively driven and no further MR data set of at least one second transmitting coil element (3) is actively driven and the first transmitting coil element (3) is not actively driven. [5] Method according to claim 1, characterized by , that to acquire the MR data set, one of the at least one second transmitting coil element (3) is actively driven. [6] Method according to claim 5, characterized by , that during the second acquisition phase the first transmitting coil element (3) is actively driven, whereby a feed-in power is iteratively changed until the MR data set is reproduced by another MR data set of at least one further MR data set. [7] Method according to any one of the preceding claims, characterized by , that the voltage values ​​that can be measured at the measuring point (5) include a forward voltage value of a forward-propagating signal component at the measuring point (5) and a backward voltage value of a backward-propagating signal component at the measuring point (5). [8] Method according to claim 7, characterized by , that the total voltage value is given by the forward voltage value, the reverse voltage value and the calibration factor. [9] Method according to one of claims 7 or 8, characterized by , that the total voltage value is given by the relationship U ~ α*U M,v + (1 / α)*U M,r is given, where U denotes the total voltage value, α denotes the calibration factor, U M,v the voltage value of the forward-propagating signal component at the measuring point (5) is designated and U M,r denotes the voltage value of the backward-running signal component at the measuring point (5). [10] Method according to any one of claims 7 to 9, characterized by , that the forward voltage value is determined by means of the computing unit (7) based on the MR data set and at least one other MR data set. [11] Method according to claim 10, characterized by , that - the reverse voltage value is determined by the computing unit (7) based on the determined forward voltage value and a predefined scattering matrix; or - to determine the reverse voltage value, the first transmitting coil element (3) is actively driven according to the determined forward voltage value and the reverse voltage value is measured at the measuring point (5). [12] Method for estimating a specific absorption rate for an object under investigation during an investigation with a magnetic resonance device (1) having a transmitter for generating an excitation field, comprising a first transmitting coil element (3) and at least one second transmitting coil element (3), wherein the first transmitting coil element (3) is coupled to the at least one second transmitting coil element (3), characterized by , that - a method for calibrating a magnetic resonance device (1) according to one of the preceding claims is carried out; - an excitation pulse is applied to the object by means of the transmitting device; - during the application of the excitation pulse, the voltage values ​​measurable at the measuring point (5) are measured; - the total voltage value is determined by means of the computing unit (7) based on the measured voltage values ​​and the calibration factor; and - the determined total voltage value is used as an input variable for a given simulation model to estimate the specific absorption rate by means of the computing unit (7). [13] Magnetic resonance device comprising a transmitter comprising a first transmitter coil element (3) and at least one second transmitter coil element (3), wherein the first transmitter coil element (3) is coupled to the at least one second transmitter coil element (3) and the transmitter device is configured to apply an excitation pulse to an object under investigation, characterized by , that - the magnetic resonance device (1) includes a measuring device (4) which is configured to measure voltage values ​​at a measuring point (5) of an electrical supply line (8) for the first transmitting coil element (3) during the application of the excitation pulse; - the magnetic resonance device (1) has a computing unit (7) which is configured to determine a total voltage value at an injection point (6) of the first transmitting coil element (3) based on the measured voltage values ​​and a predetermined calibration factor; and - the computing unit (7) is set up to use the total voltage value as an input for a given simulation model in order to determine a specific absorption rate for the object. [14] Magnetic resonance device according to claim 13, characterized by , that - the measuring device (4) has a directional coupler which is arranged on the supply line (8) in such a way as to separate a forward-propagating signal component at the measuring point (5) from a backward-propagating signal component at the measuring point (5); - the measuring device (4) includes a measuring device which is set up to measure a forward voltage value of the forward-propagating signal component and a reverse voltage value of the backward-propagating signal component in order to measure the voltage values ​​at the measuring point (5). [15] Computer program product comprising instructions which, when executed by a magnetic resonance device (1) according to one of claims 13 or 14, cause the magnetic resonance device (1) to perform a method according to one of claims 1 to 12.

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